async function async1(){
  console.log('async1 start');
   await async2();
   console.log('2.5')
   await async3();
   console.log('async1 end')
}
async function async2(){
   console.log('async2')
   await async4()
   await async5()

}
async function async3(){
  console.log('async3')
 
}
async function async4(){
  console.log('4')

}
async function async5(){
  console.log('5')

}
console.log('script start');
async1();
console.log('script end');
// 输出顺序：script start->async1 start->async2->script end->async1 end
